1. Field of the Invention
The present invention relates to a diaphragm-type miniaturized oxygen electrode, more particularly, to a miniaturized oxygen electrode useful for many applications including a measurement of the dissolved oxygen concentration of a solution, to an electrolyte composition suitable for forming the sensing site of the miniaturized oxygen electrode, and to a process of mass-producing miniaturized oxygen electrodes having a uniform quality.
An oxygen electrode is very useful for measuring the dissolved oxygen concentration in many fields. For example, oxygen electrodes are used in the field of water control, the BOD (Biochemical Oxygen Demand) in water is measured, and in the fermentation and brewing field, the dissolved oxygen concentration of a fermentation tank or fermenter is measured, to ensure an efficient fermentation of alcohol, etc.
An oxygen electrode can be combined with an enzyme to form a biosensor or an enzyme electrode to be used for measuring the concentration of sugar, vitamins, etc. For example, an oxygen electrode can be combined with glucose oxidase to measure the concentration of glucose or grape sugar. This utilizes a phenomenon in which glucose is oxidized by the dissolved oxygen with the aid of a catalytic action of glucose oxidase to form gluconolactone, with a resulting reduction of the dissolved oxygen amount diffusing into an oxygen electrode.
In addition to the measurement of the dissolved oxygen concentration of a solution, an oxygen electrode can be advantageously used for controlling the oxygen concentration of a gas phase. For example, a reduction of the ambient oxygen concentration to below 18% causes a dangerous oxygen deficiency, and in medical-care equipment, such as oxygen inhalation and gas anesthetization, the oxygen concentration of a gas used must be strictly controlled.
The oxygen electrode is thus very advantageously used in many fields, including environmental instrumentation, the fermentation industry, clinical care, and industrial hygiene.
2. Description of the Related Art
The conventional oxygen electrode typically has a structure as shown in FIG. 1, wherein a vessel or container 118 made of glass, plastics, stainless steel, or the like has an open end (lower end) covered and sealed with an oxygen gas-permeable membrane 107 made of silicone resin, fluororesin or the like, and an aqueous solution 119 of potassium chloride (KCl), sodium hydroxide (NaOH), etc., is filled in the vessel 118, in which an anode 104 made of silver (Ag), lead (Pb), etc., and a cathode 105 made of platinum (Pt), gold (Au), etc., are arranged.
The conventional oxygen electrode has a complicated structure, and therefore, it is difficult not only to miniaturize but also to mass-produce same.
The present inventors and others have proposed a new type of miniaturized oxygen electrode that can be produced by utilizing a semiconductor production process including a photolithography and an anisotropic etching, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-238,548 and U.S. Pat. No. 4,975,175.
The proposed oxygen electrode has a structure as shown in FIGS. 2 and 3, in which FIG. 2(b) shows an unfinished structure in which an oxygen gas-permeable membrane is not yet formed. This structure is produced by the following sequence. Two grooves 202 to be filled with an electrolyte-containing material are formed on a silicon wafer 201 by an anisotropic etching and the wafer surface is then covered by an SiO.sub.2 insulating layer 203 to form an electrically insulating substrate. Then, two component electrodes, i.e., an anode 204 and a cathode 205, are formed on the insulating layer 203. The anode 204 has one end 204A for external electrical connection and the other end of two branches extending into the grooves 202. The cathode 205 has one end 205A for external electrical connection and the other end extending to the top surface of a plateau retained between the grooves 202. An electrolyte-containing material 206 is filled in the grooves 202, and the filled electrolyte-containing material 206 is in contact with the anode 204 within the grooves 202 and with the cathode 205 on the plateau. The upper surface of the filled electrolyte-containing material 206 is then covered with an oxygen gas-permeable membrane 207.
Nevertheless, the step of filling the grooves 202 with the electrolyte-containing material 206 and the step of covering the filled electrolyte-containing material 206 with the oxygen gas-permeable membrane 207 are difficult to carry out in a semiconductor process, and therefore, are manually carried out chip by chip after the wafer 201 on which miniaturized oxygen electrodes have been formed is cut into chips forming respective oxygen electrodes. The manual operation is a serious obstacle to the realizing of a mass-production, and further, involves too much fluctuation in operation to obtain miniaturized oxygen electrodes having a stable or uniform performance.
Therefore, it has been desired to provide a structure of a miniaturized oxygen electrode and a production process thereof in which the filling of an electrolyte-containing material and the forming of an oxygen gas-permeable membrane can be carried out collectively or generally and uniformly, on a wafer as a whole, before the wafer is cut into chips.
The step of filling an electrolyte-containing material has the following problems.
The present inventors studied gels containing an aqueous solution of potassium chloride and polyelectrolytes and found that, because many of these are not photosensitive, the photolithography used in the semiconductor process cannot be actually applied to the filling of an electrolyte-containing material.
The electrolyte-containing material must be a liquid having a fluidity when it is filled in a groove, and the filled material must form a dense film after being dried. Also, whether or not the filled material contains water significantly affects the quality of an oxygen gas-permeable membrane applied on the filled material, and therefore, upon application for an oxygen gas-permeable membrane, the electrolyte-containing material is preferably dried. The water required for the measurement of the oxygen concentration is supplied as a water vapor through the gas-permeable membrane just before the measurement starts. The electrolyte-containing material need not contain water during the production of an oxygen electrode.
Screen printing is a preferred method of filling an electrolyte-containing material collectively in a number of miniaturized oxygen electrodes on a wafer. This screen printing generally uses an emulsion mask and a metal mask to define a printed pattern. An emulsion mask is prepared by applying a photosensitive resin in the form of an emulsion on a mesh of a stainless steel, etc. to provide a printing pattern. Some resins have a transparency which advantageously facilitates the fine alignment required when producing a miniaturized oxygen electrode because a wafer covered by a resin mask is visible through the resin. The emulsion mask, however, is very weak against water, as can be understood from the fact that the developing treatment of an emulsion is carried out by using water, and the printing of a water-containing substance is difficult. On the other hand, the metal mask is prepared by forming holes in a plate of a stainless steel, etc., and therefore, is strong against water. The metal mask, however, is disadvantageous for the fine alignment, because it does not have a transparency. Moreover, the metal mask occasionally provides a printing quality lower than that obtained by the emulsion mask, when using some kinds of printing inks.
The present inventors proposed a process in which an electrolyte-containing gel is applied by screen printing, i.e. calcium alginate gel, polyacrylamide gel, and agarose gel are printed, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-56,902. This process uses a metal mask to print an aqueous gel and cannot be advantageously used in the production of a miniaturized oxygen electrode, for the reasons mentioned above. Moreover, a strong film cannot be obtained because an oxygen gas-permeable membrane is formed on a wet gel.
Potassium chloride is generally used as the electrolyte of an oxygen electrode. Although potassium chloride is a superior electrolyte, it is not suitable for use in a miniaturized oxygen electrode because it has a drawback in that it is only soluble in water and that a filled aqueous solution becomes a white brittle powder when dried. The present inventors also proposed a polyelectrolyte, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-240,556. Although this has a good film forming property, the proposed polyelectrolyte is also soluble only in water, and is difficult to treat because it has a high polymerization degree and exhibits a high viscosity even as a dilute solution.
The step of forming an oxygen gas-permeable membrane has the following problems.
The gas-permeable membrane is made of silicone resin, fluororesin, or other electrically insulating material. The gas-permeable membrane is therefore formed not to cover the whole surface of a wafer but to have a pattern such that the component electrode ends or "pads" 204A and 205 for external electrical connection are exposed. The gas-permeable membrane is formed selectively in the predetermined wafer region other than the pad region to be exposed either by applying a resin only to the predetermined region or by first forming the gas-permeable membrane on the whole surface of a wafer and then removing the gas-permeable membrane in the pad region to be exposed.
A screen printing of a liquid resin is known as the former method, i.e., the selective application of a resin. This method has an advantage in that a single printing operation simultaneously effects both the application and the patterning of a resin, but the silicone resin used for forming a gas-permeable membrane is progressively cured by the water in the ambient air, and therefore, the viscosity of the resin varies during printing to cause a nonuniform printing, and in the worst case, a clogging of a printing stencil.
A lift-off process using a photoresist is known as the latter method, i.e., the formation and selective removal of a gas-permeable membrane. This process has an advantage in that the semiconductor process is advantageously applied and a complicated pattern can be easily obtained. This method, however, when applied in the production of a miniaturized oxygen electrode, provides a completely cured gas-permeable membrane having a high strength such that the membrane is difficult to peel or exfoliate selectively at the portion to be exposed, even by using an ultrasonic treatment. Thus, the lift-off process cannot be practically used in the production of a miniaturized oxygen electrode.
U.S. Pat. No. 4,062,750 to J.F. Butler discloses a thin film type electrochemical electrode formed on a silicon substrate, having a feature in that an electroconductive layer extends through the silicon substrate thickness so that a signal from a sensor disposed on one side of the substrate is taken out from the other side of the substrate. As this electrode does not have the pad portion of the present inventive electrode, a gas-permeable membrane may cover the whole surface and a patterning of the membrane for exposing the pad portion is not required. This electrode, however, requires a complicated production process, causing a problem in the practical application. The filling of an electrolyte is carried out by vacuum deposition, and although sodium chloride and potassium chloride can be vacuum deposited, many of the inorganic salts used as a buffering agent are deteriorated by dehydration and condensation when exposed to the heat associated with vacuum deposition. Therefore, even when a buffered electrolyte is obtained, the resulting pH will significantly deviate from an expected value and the obtained electrolyte composition must be very restricted, and thus this is not an optimum process. Moreover, problem arises when a single vacuum deposition apparatus is used for both depositing electrolytes and for depositing electrode metals, and therefore, individual deposition apparatuses must be provided for the respective depositions.
M.J. Madou et al. proposed a microelectrochemical sensor, as disclosed in U.S. Pat. No. 4,874,500 and in AIChE SYMPOSIUM SERIES, No. 267, vol. 85, pp. 7-13 (1989). This sensor also has a feature in that an electroconductive layer extends through the silicon substrate thickness and a signal from a sensor disposed on one side of the substrate is taken out from the other side of the substrate, and therefore, has the same drawback as that of J.F. Butler. An electrolyte is filled in such a manner that an alcoholic solution of poly(hydroxyethylmathacrylate), etc. is painted on, the solvent is evaporated, an electrolyte solution is introduced to form a gel, and then dried. The conventional problem is apparently eliminated, because an electrolyte is introduced after a polymer is applied, but a crystal grows when a potassium chloride solution is evaporated. When the amount of potassium chloride is small, the grown crystal is enclosed with the polymer, but when the amount is large, a number of large crystals appear, which may not be supported by the polymer. On the other hand, the amount of an electrolyte must be as large as possible, because the service life of an oxygen electrode is affected by the electrolyte amount contained therein. Thus, the restricted amount of electrolyte reduces the service life of an oxygen electrode.
A typical arrangement of the conventional oxygen electrode shown in FIG. 1, usually referred to as a "Clark type oxygen electrode", has an electrolytic solution 119 composed of an aqueous solution of potassium chloride (KCl) and an anode 104 made of silver. In this arrangement, an application of voltage across the cathode 105 and the anode 104 causes the following reactions: EQU reaction on cathode: O.sub.2 +2H.sub.2 O+4e.fwdarw.40H.sup.-( 1) EQU reaction on anode: Ag+Cl.sup.- .fwdarw.AgCl+e (2)
Oxygen of a measuring object is dissolved in the electrolytic solution, reduced on the cathode and measured in terms of a current corresponding to the amount of oxygen reduced. On the anode, the silver of the anode and the chloride ions of the electrolytic solution react (or oxidation of silver occurs) to form silver chloride and thereby are consumed.
Thus, to ensure a long period of operation of an oxygen electrode, the silver and the potassium chloride must be present in a sufficient amount.
The Clark type oxygen electrode such as shown in FIG. 1 has at least the size of a pencil and can easily contain a large amount of silver and electrolytic solution.
On the other hand, the miniaturized oxygen electrode of the present invention is as minute as some square millimeters and does not have a large volume for containing the consumable substances including the anode material such as silver and the electrolyte components such as chloride ions, for example, of KCl. Thus, the miniaturized oxygen electrode is made with an increased anode thickness and an increased concentration of the electrolyte components, to ensure the necessary amount thereof for a long period of operation.
Under normal conditions, the silver chloride formed on the anode is hard to dissolve in water and deposits on the anode, but under an increased concentration of chloride ions, the silver chloride reacts with the chloride ions to form a water-soluble complex, which is dissolved in the electrolytic solution, diffuses into the cathode and is reduced there. This means that a reducing reaction other than the reduction of oxygen as expressed by the formula (1) occurs on the cathode, and therefore, the oxygen electrode does not provide an accurate output current corresponding to the oxygen concentration of the measuring object.
Similarly, the reduction reaction of oxygen occurring on the cathode as expressed by the formula (1) yields hydrogen peroxide as an intermediate product, which diffuses through the electrolytic solution to cause an extra oxidation reaction other than that expressed by the formula (2) when reaching the anode, preventing provision of an accurate output current corresponding to the oxygen concentration of the measuring object.
The above discussion was for the case of a two-pole type oxygen electrode in which a cathode and an anode compose a set of component electrodes. The same discussion is also substantially applicable to a three-pole type oxygen electrode when assuming a cathode substituted by a working electrode and an anode substituted by counter and reference electrodes.
As described above, the miniaturized oxygen electrode has a problem in that an increased electrolyte concentration for ensuring the electrolyte amount necessary for operation causes formation of a complex of the electrolyte components on the anode or its equivalents, which are undesirablly reduced on the component electrode on which reduction of oxygen alone should occur i.e., the cathode or its equivalents, and on the other hand, an intermediate product such as OH.sup.- formed on this electrode causes an extra oxidation reaction on the other component electrodes (the anode or its equivalents), with the result that an accurate output current cannot be maintained during a long period of operation.